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1 July 2015 A Pro106 to Ala Substitution is Associated with Resistance to Glyphosate in Annual Bluegrass (Poa annua)
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Glyphosate is used in the transition zone to control annual bluegrass in fully dormant warm-season grasses. A suspected resistant (R) biotype of annual bluegrass was identified on a golf course in South Carolina after at least 10 consecutive years of glyphosate application. Greenhouse bioassays revealed the R biotype was 4.4-fold resistant to glyphosate compared with a standard susceptible (S) biotype. Further studies were conducted to investigate the mechanism conferring glyphosate resistance in the R biotype. Leaf discs of both biotypes accumulated shikimate in response to increasing glyphosate concentration, but the glyphosate concentration resulting in 50% EPSP synthase inhibition as a result of shikimate accumulation (I50) was 4.2-fold higher in the R biotype compared with the S biotype. At the whole plant level, similar levels of shikimate accumulation were observed between biotypes at 6 and 24 h after treatment (HAT) with glyphosate, but greater shikimate accumulation occurred in the S biotype at 72, 120, and 168 HAT. Shikimate levels decreased in the R biotype after 72 HAT. There were no differences in 14C-glyphosate absorption between biotypes. However, more 14C-glyphosate translocated out of the treated leaf in the R biotype and into root tissues over time compared with the S biotype. Partial sequencing of the EPSP synthase gene revealed a point mutation that resulted in an Ala substitution at Pro106. Although other mechanisms may contribute to glyphosate resistance, these results confirm a Pro106 to Ala substitution is associated with resistance to glyphosate in the R annual bluegrass biotype.

Nomenclature: Glyphosate; EPSP, 5-enolpyruvyl-shikimate-3-phosphate; shikimate, (3R,4S,5R)-3,4,5-trihydroxycyclohex-1-ene-1-carboxylic acid; annual bluegrass, Poa annua L.

© 2015 Weed Science Society of America
Robert B. Cross, LAMBERT B. McCARTY, Nishanth Tharayil, J. Scott McElroy, Shu Chen, Patrick E. McCullough, Brian A. Powell, and WILLIAM C. BRIDGES Jr. "A Pro106 to Ala Substitution is Associated with Resistance to Glyphosate in Annual Bluegrass (Poa annua)," Weed Science 63(3), 613-622, (1 July 2015).
Received: 3 March 2015; Accepted: 1 March 2015; Published: 1 July 2015

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